1. Technical Field
The present invention relates to an apparatus for measuring a user's physiological information, such as a heart rate.
2. Related Art
Since a heartbeat manifests itself as a pulse, i.e. a change of volume of blood, it is possible to measure the heart rate using a photoelectric pulse sensor that detects a change in blood volume at a site on a user's body, e.g. a wrist. However, the volume of blood also changes depending on body movements in addition to heartbeats. For this reason, when measuring the heart rate using a photoelectric pulse sensor, noise caused by the body movement is included in the process of transmission of the blood from the heart to the wrist. That is, since blood is a fluid and the blood vessels are elastic, the flow of blood generated by body movements causes a change in an amount of blood, the flow being measured as a pseudo-pulsation.
A heart rate monitor that accounts for a noise component due to body movements is disclosed in JP-A-55-120858, which is incorporated by reference herein. Light beams with different wavelengths are transmitted, reflected, and measured. Pulsation components are calculated from the measured values. The method uses the fact that oxygenated hemoglobin (which is dominant in arterial blood) and reduced hemoglobin (which is dominant in venous blood) have different light absorption characteristics.
However, the light beams of different wavelengths also penetrate the user's skin to different depths. This causes additional noise, making it difficult to remove the noise caused by the body movement.